Audio amplifiers are a class of signal amplifiers used in a wide variety of electronic equipment, including stereo audio systems, televisions, personal media players, and cellular phones. Some of these amplifiers drive speakers, such as those in a stereo audio system or television, while others typically drive headphones, such as those used with personal media players and cellular phones.
In many cases it is desirable to limit the peak or average audio output level delivered to headphones, to preclude hearing loss from excessive sound pressure levels. Many devices driving headphones, such as compact disc players, MP3 players, and portable DVD players incorporate circuitry to limit audio output levels delivered to headphones.
One known approach is the use of an automatic gain control (AGC), which compares the peak or average audio level to a threshold, and decreases the amplifier gain so as to reduce output level for some time period after this threshold is exceeded. Such circuits typically have time constants with relatively fast attack and much slower decay to reduce audible effects of the AGC operation. To minimize the undesired modulation of the audio level by the AGC, sometimes referred to as “pumping”, the time constant for decay is often seconds long. This long time constant often leads to an undesirable characteristic wherein a brief sound above the threshold level, for example a loud cymbal, causes a rapid reduction in output level, not only of the brief sound which exceeded threshold but of all sound for the next several seconds. Following the over-threshold audio, if the audio levels remain below threshold sound level, the amplitude of the audio will gradually increase, at a rate set by the decay time constant, until gain is again at its nominal level. This gain change during music leads to undesired distortion of the dynamic range of the audio, and also may cause soft music following a loud musical peak to be so low in level as to be difficult to hear.
Another known approach to limiting the output level of an audio amplifier is the use of clipping of the audio signal on a cycle by cycle basis. A clipper simply replaces any portion of an audio waveform which otherwise would be above a threshold level with a constant voltage at the threshold level, until the waveform voltage again falls below threshold. There is typically no intentional gain change caused by such clipping, and the time constant for clipping is typically much faster than the period of a single cycle of audio, even at its highest frequencies.
Clipping therefore avoids the dynamic range modulation artifacts or “pumping” associated with an AGC, and causes little or no change in audio levels which stay below the clipping level. However, clipping of those portions of the waveform which otherwise would have exceeded the threshold level causes significant increases in harmonic distortion, due to the introduction of abrupt discontinuities in the audio waveform at the start and end of clipping for each cycle.
Some examples of prior art devices can be seen in U.S. Pat. Nos. 3,737,678; 3,818,244; 5,737,432; and 7,155,020.
An apparatus and method for limiting the output level of an audio amplifier, which causes less dynamic range modulation than an AGC and less harmonic distortion than known clipping circuits, is therefore desirable, and is an object of the present invention.